n-Doped electrically-conductive polymers are π-conjugated polymer organic semiconductors (OSCs) that have been n-doped to an electrically-conductive state. This can be achieved using an n-dopant, which is a strong reductant or electron donor, introduce electrons into the π-conjugated system of the polymer backbone. The electrons are mobile and can therefore carry current. As the polymer backbone becomes negatively-charged, it needs to be counterbalanced by cations called counter-cations. Although molecular OSCs can be n-doped, n-doped polymer OSCs have particular advantages related to solution and film processing to give electron-injection layers (EILs) and electron-extraction layers (EELs).
n-Doped polymer OSCs are useful as electron contacts of semiconductor devices to perform the function of electron injection into semiconductors for various device applications, and of electron extraction in solar-cell and photovoltaic applications. The layers performing these functions are called EILs and EELs, respectively. A suitable electrical conductivity for applications as EILs and EELs in semiconductor devices is between 10−6 to 102 S cm−1. In practice, however, the n-doped polymer OSCs that have been achieved so far are unstable. As a result, n-doped electrically-conductive polymers are not available for EIL and EEL application development even on a research scale. The key challenges are poor chemical, thermal and processing instability of negatively-charged π-conjugated systems, restricted thermodynamic window of stability for n-doped polymers in ambient, and general difficulty to form films with stable doping profiles (i.e., the dependence of doping level with distance across the layer thickness) that do not change deleteriously with processing.
Besides electrical conductivity, the workfunction (WF) of the EIL and EEL also plays a role in determining its effectiveness as an electron contact for electron injection and extraction. When the EIL or EEL is applied in a device, what matters is the effective workfunction (WFeff) of the EIL (EEL) at its buried contact with the adjacent semiconductor inside the device. The energy difference between the WFeff of the EIL and the electron affinity (EA) of the semiconductor gives the apparent thermodynamic barrier for electron injection (Δe) from the EIL into the semiconductor. However, this WFeff is related to the vacuum WF which is a property of the EIL or EEL film.
Polymer OSCs can be n-doped by evaporation of low-workfunction metals, for example the alkali metals (Li, Na, K, Rb and Cs) and alkaline-earth metals (in particular Ca and Ba) and some transition metals (Sm) onto their surfaces. The resultant n-doping of the polymer surface is confirmed by changes in the density-of-states of the polymer OSC which can be measured by ultraviolet photoemission spectroscopy, and also validated through the greatly improved electron-injection currents observed in diodes (see for example, U.S. Pat. No. 8,049,408). This introduces the corresponding metal cation as the counter-cation. However, the doping only occurs at the surface. Furthermore, these are not generally stable against dedoping for manipulation into devices, nor against migration of the doping profile, i.e. doped carrier density as a function of distance, which frustrates the development of a general solution-based approach for ohmic contacts employing doped polymers. Alternative approach employing metal oxides has limitations including the need for vacuum deposition and/or high temperature post-annealing which may not be desirable.
n-Doped polymers are different from “n-type” polymers. Examples of n-type polymers include insulating and semiconducting polymers such as: polyethylenimine ethoxylated (PEIE) and polyethylenimine (PEI) that are deposited onto transparent conducting oxides and p-doped conducting polymers such as poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) to give low WF surfaces; and undoped conjugated polyelectrolytes. However these do not have well-defined bulk WF, are highly resistive and cannot generally give ohmic electron contacts to semiconductors.
n-Doped polymers may also be made by self-n-doping using hydroxide or iodide as dopant. However, this self-n-doping is generally not able to provide a doping level of more than 0.1 electron per electron-deficient moiety and a WF shallower than 4.0 eV needed to achieve effective EIL (EEL) materials.
There is therefore a need for an improved material capable for use as EIL or EEL.